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This paper evaluates model-based upgrading of an activated sludge plant treating wastewaters of leather tanning industrial district with an average capacity of 13,000 m 3 /d. Significant impact of temperature changes on biochemical processes, such as nitrification and denitrification were quantified and used in setting the aeration control for process optimization in terms of total nitrogen removal. Model evaluation based upon kinetic and stoichiometric parameters specifically determined for the tannery wastewater, indicated that an intermittent aeration type of an operation strategy was suitable for achieving the required effluent total nitrogen level of 100 mgN/L under all operating conditions at the process temperature range of 21°C-35°C. The simulation study also showed that the nitrification performance of the plant was improved by increasing the aeration intensity. The application of intermittent aeration provided a competitive advantage in meeting the total nitrogen standards, improving the total nitrogen removal to around 60%.

Some agro-industries generate wastewaters with very high COD concentrations and require two-stage biological treatment Inert or residual organic constituents of these wastewaters, even at very low ratios, play a major role in the compliance of effluent standards. In this study, two strong wastes, citric acid plant effluent and cheese whey were tested for their inert COD content. The experiments showed that the first-phase anaerobic treatment provided a major decrease in the residual COD, but introduced significant amounts of particulate residual COD to the second phase. A newly developed procedure was used for the assessment of the soluble and particulate residual COD.

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Textile effluents often contain an array of chemicals with different biodegradation characteristics. Consequently, it is quite difficult to evaluate and interpret the degree of COD removal that can be attained by biological treatment without identifying COD portions that are resistant to biodegradation. This study evaluates the biological treatability of textile wastewaters generated by the knit and wowen fabric finishing category with specific emphasis on the assessment of different residual COD components. A new method is proposed to experimentally measure the initial particulate inert COD. The method is tested to yield a value of 73 mgl-1 for this COD component, corresponding to 16 % of the particulate COD in the textile sample. A previously developed procedure is used to quantify the initial soluble inert COD and the residual COD generated through microbial metabolism during the treatment process. The ratio of the inert fraction to the soluble COD of the textile effluents is found to vary between 0.076 and 0.22. A similar ratio in the range of 0.04 - 0.09 is calculated for the residual microbial products. The kinetic and stoichiometric constants associated with the biodegradable COD are also experimentally measured. The residual components, together with the kinetic information about biodegradable fractions, are used to simulate the performance of activated sludge systems by means of a relationship between the total effluent COD and the sludge age. The results indicate that the residual components practically dominate the effluent COD and seriously challenge related effluent standards.